The overall objective is to determine the interaction of factors involved in control of iliac artery blood flow (IBF), superior mesenteric artery blood flow (SMBF) and heart rate (HR) in unanesthetized baboons during external heating and how this interaction is altered during a fever. One set of experiments is designed to determine the role of skin temperature (Ts) and core temperature (Tc) changes in effecting changes in IBF, SMBF, and HR during heat stress. Independent control of Tc will be obtained by use of a heat exchanger incorporated into a chronic femoral arteriovenous (A-V) shunt; whereby Tc can be elevated independently of Ts changes at both cool and warm Ts levels, and Tc can be kept constant during Ts elevation by external heating. By these combinations of Tc and Ts, the relative importance of Ts vs. Tc in effecting the changes in IBF, SMBF, and HR, and the influence of Ts changes of cardiovascular variable-Tc relationships (i.e., altered slope and/or set-point?) will be determined. A second set of experiments will focus on how fever alters the cardiovascular variable-Tc relationships during Tc elevations. Tc will again be controlled by a heat exchanger in a femoral A-V shunt. With the baboon in a neutral environment, Tc will be raised and lowered before pyrogen injection and during the plateau phase of the fever following pyrogen injection. The cardiovascular variable-Tc relationships before and during fever will be examined for changes in slopes and set-points. The third set of experiments will seek to determine the involvement of the autonomic nervous system in effecting the changes in HR and SMBF during heat stress and fever. Conventional vagal, alpha-receptor, and beta-receptor blocking drugs will be used to determine if they alter the HR and SMBF responses during heat stress and fever.